The present invention relates to an apparatus and method for determining liquid level in a steam drum or similar vessel operating at or near saturated conditions, and, more particularly, an apparatus and method that allows for a more accurate reading of the liquid level in a steam drum or similar vessel by compensating for variances in operating pressures and temperatures.
The American Society of Mechanical Engineers (“ASME”) has promulgated a Boiler Code, which, in pertinent part, states that “[a]t least one gage glass shall be readily visible to the operator in the area where control actions are initiated. Alternately, two dependable indirect indications shall be provided, either by transmission of the gage glass image or by remote level indicators . . . ” ASME Boiler Code, Section I, Part PG-60.1.1.
Since it is often difficult to have a gage glass in the immediate vicinity of the control room, the alternate option is typically preferred, wherein two indirect indications of liquid level in a steam drum or similar vessel are provided. In this regard, a master control system typically serves as the first indirect means of indication. The industry standard for power plants has been the use of a water column with conductivity probes as the second means of remote level indication (“RLI”).
Using a water column and conductivity probes, however, does not always provide the desired measurement accuracy. Indeed, it is common that the water column and conductivity probe arrangement provides measurements that are in disagreement with the computerized master control system, since the probe method is only accurate for one set of operating conditions.
FIG. 1 is a schematic view of a standard prior art water column with conductivity probes. As shown, the water column 12 is secured to and in liquid communication with a steam drum 10. As the liquid level rises or falls in the steam drum 10, the liquid level simultaneously rises or falls in the water column 12. To transmit this liquid level in the water column 12 to a remote location (i.e., the control room), a series of conductivity probes 14 are inserted into the water column to sense the liquid level. A display unit 16 in the remote location has a series of light indicators, each of which correspond to a specific conductivity probe. Therefore, for each conductivity probe 14 that is in contact with liquid, a corresponding indicator of the display unit 16 is illuminated.
However, such a method of remote level indication is rife with error as a standard water column with conductivity probes is only accurate at a single predefined set of operating conditions. Specifically, the density of the liquid in the water column 12 is not always the same as that of the liquid in the steam drum 10, especially at elevated operating temperatures and pressures. In this regard, as shown in FIG. 1, the conductivity probes 14 are essentially switches that are installed at fixed points along the water column 12. Thus, to take into account density differences would require relocation of the conductivity probes 14 relative to the water column 12. To address this problem, some attempts have been made to provide conductivity probes that can be adjusted to account for density differences through a range of operating conditions. However, since an appropriate adjustment would be required each time the operating conditions are altered, such a solution has not proved satisfactory.
There is therefore a need for an apparatus and method for determining liquid level in a steam drum or similar vessel that overcomes the problems of the prior art, an apparatus and method that takes into account liquid density differences between a steam drum or similar vessel and a water column used for determining the level of liquid in the steam drum or similar vessel.